This invention relates to a system for remotely detecting the failure of individual lamps in a multilamp lighting system, and more particularly to a system for detecting the failure of such lamps in a high intensity approach lighting system for an airport.
Many airports today, particularly those having Category II runways under Federal Aviation Administration (FAA) classification criteria, are equipped with a dual mode high intensity approach lighting system. Such a system provides visual approach lighting patterns to landing aircraft, and typically has a first high intensity approach lighting system mode and a second simplified short approach lighting system mode. The typical system is capable of providing 3000 foot patterns with any needed glide slope angle restrictions, and also shorter 2400 foot patterns for use on other domestic Category II runways, and includes both steady burning approach lights and sequenced flashing lights to provide directional guidance to the approaching aircraft. The steady burning approach lights are connected in a number of constant current lighting loops, and may be operated at several, for example, five, distinct brightness levels. Switching between the modes may be locally controlled from an adjacent substation, or remotely controlled from the air traffic control tower via a control subsystem.
In the first high intensity approach mode, the typical system includes approximately 100 lamps of the 300 to 500 watt type, connected in series in each of a plurality, for example, three, constant current loops. Additionally, fifteen flashers are active in a typical embodiment, so that the sequence will begin with the flasher farthest from the threshold of the runway and proceed toward the flasher closest to the runway threshold. Of course, the actual number of lamps and lamp wattage may vary for each loop of steady burning lights, and the number of flashers may vary for a specific application.
In the second simplified short approach mode, approximately 18-25 steady burning 300-500 watt lamps are employed, while approximately every other flasher employed in the first mode is used in the second mode, so that the time interval between flashes of a single sequence in the second mode is about twice that in the first mode.
FIG. 1 is a system block diagram illustrating a typical prior art dual mode approach lighting system 11. The system 11 includes equipment packages in three geographic locations, including a runway equipment package 13, a substation equipment package 15, and a control tower equipment package 17. The runway equipment package 13 includes steady burning lights 19 (preferably of the PAR-56 type) which are arranged in a predetermined pattern in the runway approach zone. Also included as part of the runway package are isolation transformers, junction boxes, an aiming device, flasher light units and control cabinets, and lampholders and shorting devices for each of the lights 19. The substation equipment package 15 includes a substation control and monitor 21, a high voltage input cabinet 23, a high voltage output cabinet 25, constant current regulators 27, 29, and 31 for regulating the current on each of the three lighting loops, a flasher master control cabinet 33, and a transformer and distribution panel 35. All of this equipment is preferably located in a substation building positioned near the end of the runway. Finally, the control tower equipment package 17 comprises a remote control panel 37 and a remote electronics chassis 39.
In operation, the remote control panel 37 permits control of the lighting system 11 from the control tower, and displays the operational status of the system. The remote electronics chassis 39, in the control tower, receives control and status signals through a telephone line 41 from the substation and processes them for display on the remote control panel 37. In turn, control signals from this panel are routed through the telephone line 41 to the substation. The high voltage input cabinet 23 receives high voltage three-phase input power from a power source through a power line 43 to operate the steady burning lights and flashing lights. AC input from the high voltage input cabinet 23 is provided to the three constant current regulators 27, 29, and 31 through a power line 45, as well as to the transformer and distribution panel 35 through a power line 47, and to the substation control and monitor 21 through a power line 49. The constant current regulators 27, 29, and 31 are used to power the three lighting loops. Each regulator can supply 50 kW output at 20 amperes. The regulators preferably have five intensity steps: 8.5 A, 10.3 A, 12.4 A, 15.8 A, and 20 A. The regulators can be operated by their own local control panel or remotely from the substation control panel 21 or the air traffic control tower control panel. Monitoring circuits detect the actual current flowing into the regulator output and supply intensity status signals to the control and monitor cabinet 21.
The high voltage output cabinet 25 distributes the output of the constant current regulators, which is received via power line 51, to the lighting loops, via power line 53. Each regulator output has a shorting disconnect to short both the regulator output and the lighting loops circuit during maintenance. Three high voltage relays, one for each lighting loop, switch to a portion of the lighting loops in the simplified short approach mode. The relays are controlled from the control and monitor cabinet 21. A monitoring bank of isolation transformers and reference lights 55 monitor the regulator output voltage by means of a sampling line 57, in a manner to be described more completely hereinbelow.
Other important system components include the flasher master controller 33, which controls operation of the sequenced flashers. The flashers can be operated from the control panel or remotely through the control and monitor cabinet 21. The master controller 33 can monitor the status of the flasher light units, as well as control the intensity thereof. A shorting device maintains the integrity of the 20 amp series circuit when a lamp burns out, by providing a short circuit around that non-functional lamp. One is located in each lampholder assembly. The substation control and monitor cabinet 21 contains the control and monitor circuitry to operate the flashing light system and the steady burning system. The control panel within the control and monitor cabinet 21 monitors the input voltage, input power, and regulator output voltages by use of meters. Control switches place the system into operation, select light intensity levels, and select between the two available modes. The panel displays system cautions and warnings and selection of local or control tower operation of the system. The switches on the control panel have integral lights. The lights work independently of the switch, so that when a switch is activated and a signal is received by the equipment, the equipment returns a signal turning on the light. For example, when brightness level 3 is selected, the brightness 3 indicator light will not illuminate until the constant current regulators 27, 29, and 31 return the signal that indicates they are operating at an intensity 3 current level.
The control and monitor cabinet 21 houses two racks of circuit card assemblies that contain the electronics required to perform control and monitor functions. These circuit card assemblies include a monitor alarm circuit board assembly which controls the monitoring process, as well as three monitor channel circuit board assemblies, one for each of the three lighting loops. The primary functions are:
a) to provide brightness selection of the lamps. The steady burning lamps operate on brightness 1 level for a short duration before stepping to the selected intensity. When brightness 5 is selected, the lights are automatically reduced to brightness 4 after a predetermined period of time; PA1 b) to turn off the constant current regulators while changing modes in order to allow the high voltage relays in the high voltage output cabinet 25 to transfer without being loaded; PA1 c) to transmit by telephone line 41 digital control and status signals to the control tower remote control panel, and to receive by telephone line digital control signals from the tower to operate the system; and PA1 d) to monitor the steady burning lights and detect lamp failures. This is accomplished by monitoring reference lamp voltage in each lighting loop as well as each loop voltage. Reference voltage signals from isolation transformers and reference lights 55 are passed to and monitored at control and monitor cabinet 21. A change in loop voltage caused by one or more failed lamps is detected and compared with the reference lamp voltage. When the number of failed lamps exceeds a preset number, a caution or failure signal is displayed on the control panels and an audible alarm is sounded. The monitor test panel is contained in the card racks. The meter on the panel provides a visual indication of the number of failed lamps. Screwdriver adjusted variable resistors located on the panel are preset to a known number of failed lamps and can be switched into the monitor circuitry to check their operation without disturbing the lighting field lamps.
The problem with the lighting system, as described above, is that the lamp failure detection method relies on the monitoring of the voltage level on each of the three lighting loops. This method of failed lamp detection has proven to be unsatisfactory for a number of reasons. The major problem is that this approach is very susceptible to the condition of the lighting loop cabling. Change in cable conditions caused by unavoidable occurrences such as changes in temperature, humidity, deterioration in insulation, and increase in the resistance across cable connections, as well as lamp aging, all result in incorrect detection of lamp failures. The reason for this is that the changes in cable insulation and corrosion of electrical connections result in an increase in the impedance of the loop. The constant current regulators 27, 29, and 31 correct this condition by increasing the regulator output voltage to maintain a constant current through the lighting system (of course, the ability of the system to compensate for this degradation is limited by the 50 kW capability of the constant current regulators). The failure of a lamp triggers the activation of the shorting device in the particular lamp holder, thereby reducing the total voltage across the affected lighting loop. However, the fact that the system voltage is continually adjusted by the constant current regulators in order to maintain a constant current across the loop as it degrades, will obviously diminish the ability of the monitoring circuits to detect the change in loop voltage caused by the lamp failure.
Another problem with the lamp failure detection system presently in use is the lengthy time required for installation and calibration. Approximately 73 potentiometer adjustments are required each time the system is installed or calibrated. Furthermore, even when the system is operating properly, only very limited information is available, i.e. that a threshold number of lamps have failed on one of the three lighting loops. No indication as to which specific lamp has failed is given. Calibration of the state of the art system is so cumbersome and time-consuming, and it remains calibrated for such a brief period of time, that some airports have disabled the monitoring circuitry and have resorted to a manpower intensive hourly visual monitoring of the light system.